1. Field of Application
The present invention relates to an electronic control apparatus, and in particular to an electronic control apparatus which controls driving of an electrical load by on/off switching of a MOS FET (metal-oxide semiconductor field effect transistor), such as an N-channel MOS FET that is connected to controllably apply a power supply voltage to a load.
2. Related Art
In the prior art, it is known to provide an electronic control apparatus in a motor vehicle, for use as an engine control apparatus or transmission control apparatus, which controls operations of the vehicle engine or of the vehicle transmission by controlling relays, solenoids, etc. However in recent years, with the advances made in automatic control, there has been an increasing trend for requiring a greater amount of electrical load to be driven by such an electronic control apparatus. That is to say, due to the demand for a higher degree of accuracy and flexibility of controlling various controlled mechanisms, the amount of electrical load that must be driven has increased. In addition, there has been a broadening of the range of functions that are implemented by electronic control.
MOS FETs are primarily used as switching elements for performing switching whereby power is selectively supplied to an electrical load, due to the fact that MOS FETs can carry a higher level of current with a lower amount of heat being generated than is possible with bipolar transistors. In particular, N-channel MOS FETs are widely used, since these are less expensive than P-channel MOS FETs and generate lower amounts of heat.
In the case of electrical equipment of a motor vehicle, it is preferable that “high side” driving of switching elements is performed, i.e., whereby the switching element is located upstream with respect to an electrical load, in order to provide greater safety in the event of a short-circuit occurring whereby an electrical path is produced between the electrical load and ground potential, that is to say, 0 V potential. The descriptions given herein assume the use of a power source having one terminal connected to ground potential, and the term “located upstream” is used herein with the significance of being connected between a first terminal of an electrical load and the power source potential, with the second terminal of the electrical load being connected to ground potential.
For that reason, with an electronic control apparatus used in a motor vehicle, the electronic control apparatus controls on/off switching of an N-channel MOS FET that is connected at the upstream side of an electrical load, in series with that load. The switching is generally performed in accordance with command signals that are produced from a microcomputer. In addition, such an electronic control apparatus generally includes a voltage step-up circuit for producing a stepped-up voltage that is higher than the power supply voltage of the electronic control apparatus, with that stepped-up voltage being selectively applied through a pre-drive circuit to the gate of the N-channel MOS FET, in accordance with the command signals, to achieve the on/off switching. In the case of an electronic control apparatus used in a motor vehicle, the power supply voltage is generally that of the vehicle battery. Such types of electronic control apparatus are described for example in Japanese Patent Laid-open No. 4-241511 and Japanese Patent No. 2572408. Typically, a charge pump type of circuit is used as the voltage step-up circuit, whereby charging of a plurality of capacitors is used to achieve the stepped-up voltage. Such types of electronic control apparatus are described for example in the aforementioned Japanese Patent Laid-open No. 4-241511 and Japanese Patent No. 2572408, and also in Japanese Patent No. 3314473 and Japanese Patent No. 23368783.
Generally, in order to achieve compactness of the electronic control apparatus, the voltage step-up circuit and the pre-drive circuit are configured in an IC (integrated circuit). This may be separate from one or more N-channel MOS FETs that are driven thereby. Alternatively, the voltage step-up circuit and the pre-drive circuit may be configured together with the N-channel MOS FET, as an IC.
A configuration for an electronic control apparatus that has been previously envisaged by the assignee of the present invention will be described referring to the circuit example of FIG. 6. In the following and in the description of embodiments given hereinafter, the terminal “voltage” is to be understood as signifying a DC voltage, unless otherwise indicated. In FIG. 6, an electronic control apparatus 100 is connected to one terminal of each of five solenoids L1 to L5, which constitute the electrical load of the electronic control apparatus 100. The opposite terminal of each of the solenoids L1 to L5 is connected to ground potential (GND).
The electronic control apparatus 100 includes a microcomputer 7 which determines the respective levels of power that are supplied to drive the solenoids L1 to L5, a driver IC 101 that drives the solenoids L1, L2 in accordance with command signals S1, S2 respectively that are part of a set of command signals S1 to S5 produced from the microcomputer 7, a driver IC 102 that drives the solenoids L3, L4 in accordance with the command signals S3, S4 respectively from the microcomputer 7, and a driver IC 103 that drives the solenoid L5 in accordance with the command signal S5.
In FIG. 6, the concentric-circle symbols indicate respective terminals of the electronic control apparatus 100, and the single-circle symbols indicate terminals of the driver ICs 101 to 103. This convention is also used in other drawings that are described in the following.
The driver ICs 101 to 103 are internally provided with respective charge pump circuits 111 to 113, each of which generates a stepped-up voltage that is higher than the power supply voltage, constituted by the voltage VB of the battery 9.
In addition to the charge pump circuit 111, the driver IC 101 internally incorporates an N-channel MOS FET 1, with the source electrode connected to the opposite terminal of the solenoid L1 from the ground terminal thereof and the drain electrode connected to receive the battery voltage VB, an N-channel MOS FET 2 with the source electrode connected to the opposite terminal of the solenoid L2 from the ground terminal thereof and the drain electrode connected to receive the battery voltage VB, and two pre-drive circuits 11 and 12 which each receive the stepped-up voltage produced from the charge pump circuit 111, transferred via a diode 21 and are controlled by the command signals S1, S2 respectively from the microcomputer 7. When the command signal S1 is at the active level (assumed herein to be a high level), the pre-drive circuit 11 applies the stepped-up voltage from the charge pump circuit 111 to the gate electrode of the N-channel MOS FET 1, thereby turning the N-channel MOS FET 1 on (i.e., to produce a conducting path between drain and source), while when the command signal S1 is not at the active level, the pre-drive circuit 11 does not apply the stepped-up voltage to the gate electrode of the N-channel MOS FET 1, thereby turning the N-channel MOS FET 1 off. In the case of the pre-drive circuit 12, in the same way as for the pre-drive circuit 11, when the command signal S2 is at the active level, the pre-drive circuit 12 applies the stepped-up voltage from the charge pump circuit 111 to the gate electrode of the N-channel MOS FET 2, thereby turning the N-channel MOS FET 2 on, while when the command signal S2 is not at the active level, the pre-drive circuit 12 does not apply the stepped-up voltage to the gate electrode of the N-channel MOS FET 2, thereby turning the N-channel MOS FET 2 off.
Similarly, in addition to the charge pump circuit 112, the driver IC 102 internally incorporates an N-channel MOS FET 3, with the source electrode connected to the opposite terminal of the solenoid L3 from the ground terminal thereof and the drain electrode connected to receive the battery voltage VB, an N-channel MOS FET 4 with the source electrode connected to the opposite terminal of the solenoid L4 from the ground terminal thereof and the drain electrode connected to receive the battery voltage VB, and two pre-drive circuits 13 and 14 which each receive the stepped-up voltage produced from the charge pump circuit 112, transferred via a diode 22, and are controlled by the command signals S3, S4 from the microcomputer 7 respectively. When the command signal S3 is at the active level, the pre-drive circuit 13 applies the stepped-up voltage from the charge pump circuit 112 to the gate electrode of the N-channel MOS FET 3, thereby turning the N-channel MOS FET 3 on, while when the command signal S3 is not at the active level, the pre-drive circuit 13 does not apply the stepped-up voltage to the gate electrode of the N-channel MOS FET 3, thereby turning the N-channel MOS FET 3 off. Similarly when the command signal S4 from the microcomputer 7 is at the active level, the pre-drive circuit 14 applies the stepped-up voltage from the charge pump circuit 112 to the gate electrode of the N-channel MOS FET 4, thereby turning the N-channel MOS FET 4 on, while when the command signal S4 is not at the active level, the pre-drive circuit 14 does not apply the stepped-up voltage to the gate electrode of the N-channel MOS FET 4, thereby turning the N-channel MOS FET 4 off.
Moreover in addition to the charge pump circuit 113, the driver IC 103 internally incorporates an N-channel MOS FET 5, with the source electrode connected to the opposite terminal of the solenoid L5 from the ground terminal thereof and the drain electrode connected to receive the battery voltage VB, and a pre-drive circuit 15 which receives the stepped-up voltage produced from the charge pump circuit 113, transferred via a diode 23 and is controlled by the command signal S5 from the microcomputer 7. When that command signal is at the active level, the pre-drive circuit 15 applies the stepped-up voltage from the charge pump circuit 113 to the gate electrode of the N-channel MOS FET 5, thereby turning the N-channel MOS FET 5 on, while when the command signal S5 is not at the active level, the pre-drive circuit 15 does not apply the stepped-up voltage to the gate electrode of the N-channel MOS FET 5, thereby turning the N-channel MOS FET 5 off.
It can thus be understood that when a command signal Sn (where n is in the range 1 to 5) is produced from the microcomputer 7 at the active level, the FETn that corresponds to the command signal Sn is set in the on state, whereby current flows through the corresponding solenoid Ln.
As shown in FIG. 6, in the driver ICs 101 to 103, respective capacitors 24, 25, 26 are connected between a power supply terminal Td (to which the battery voltage VB is applied) and the ground terminal Tg of each driver IC. In addition, an inductor (i.e., coil) 27 is connected in series between the power source terminal Td of the driver IC 101 and a portion 28 of a printed circuit pattern in the electronic control apparatus 100 that transfers the battery voltage VB (that portion being referred to in the following as the common power supply lead). The capacitors 24 to 26 and the inductor 27 serve as noise suppression elements, to suppress electrical noise that is generated by the operation of the charge pump circuits 111 to 113.
The action of these noise suppression elements will be described referring to a specific example. FIG. 7A shows an example of a charge pump circuit 30, which is representative of each of the charge pump circuits 111 to 113 shown in FIG. 6, while FIG. 7B shows waveforms for describing the operation of the circuit of FIG. 7A.
The charge pump circuit 30 is made up of a discharge-operation switching element 31 formed of a PNP transistor having its emitter connected to the power source terminal Td and a charge-operation switching element 32 formed of an NPN transistor having its collector connected to the collector of the discharge-operation switching element 31 and its emitter connected to the ground terminal Tg, a diode 33 having its anode connected to the emitter of the discharge-operation switching element 31, a diode 34 having its anode connected to the cathode of the diode 33, a capacitor (charge pump capacitor) 35 for performing voltage step-up, connected between the collectors of each of the switching elements 31, 32 and the junction of the diodes 33, 34, and an output capacitor 36 which is connected between the cathode of the diode 34 and the ground terminal Tg. The charge pump circuit 30 further includes a control signal generating section 37, for alternately setting the switching elements 31, 32 periodically on and off, as illustrated in FIG. 7B. The control signal generating section 37 incorporates an RC (resistor-capacitor) type of oscillator circuit (not shown in the drawings), made up of resistors, capacitors, etc., with the oscillation period of that oscillator circuit determining the period of on/off switching of the switching elements 31 and 32.
When the charge-operation switching element 32 of the charge pump circuit 30 is set in the on (i.e., conducting) condition, then as illustrated by the double-dot chain line arrows in FIG. 7A, current flows along a path from the power source terminal Td via the diode 33 through the capacitor 35, then through the charge-operation switching element 32, to the ground terminal Tg, thereby charging the capacitor 35. When the discharge-operation switching element 31 is set on, then as illustrated by the single-dot chain line arrows in FIG. 7A, current flows along a path from the power source terminal Td through the discharge-operation switching element 31, then through the capacitor 35, then through the diode 34, and through the output capacitor 36, to the ground terminal Tg, thereby discharging the capacitor 35 and charging the output capacitor 36.
As a result of successive repetitions of these operations, the charge voltage of the capacitor 35 is added to the charge voltage of the capacitor 36, so that a stepped-up voltage appears across the output capacitor 36 and is produced as the output voltage from the charge pump circuit 30. Due to the fact that a capacitor C is connected between the power source terminal Td and the ground terminal Tg, as shown in FIG. 7B, each time a switching element 31 or 32 is switched from the off to the on condition, current flows from the capacitor C towards the power source terminal Td. This prevents a sudden increase in current flow through the common power source line 28, as illustrated in the lower diagram of FIG. 7B.
Hence, by using the capacitor C, abrupt increases in the current that flows into the power source terminal Td from the common power source line 28 are suppressed, so that current is supplied from the common power source line 28 to the charge pump circuit 30 in a comparatively gradual manner. As a result, generation of electrical noise due to variations in current and voltage of the common power source line 28 can be effectively reduced.
If the capacitor C were not connected, then variations of voltage and current due to charging and discharging of the capacitor 35 in the process of generating the stepped-up voltage would have a significant effect on the common power source line 28, which supplies the battery voltage VB. Hence, electrical noise would be generated in the wiring harness which supplies the battery voltage VB to the electronic control apparatus 100, and this noise might affect a radio or other apparatus of the vehicle. In general, such variations in current result in electrical noise that is in a relatively low frequency range, whereas the variations in voltage result in electrical noise that is in a relatively high frequency range. The circuit configuration shown in FIG. 7A is a basic circuit, having only a single-stage capacitor 35 (i.e., charge pump capacitor) that is charged and discharged, however a greater number of such capacitors may be utilized, in accordance with the required value of stepped-up voltage and the minimum value of power source voltage of the circuit. Specifically, if the circuit is operated in a no-load condition, the output voltage level is determined by the power source voltage and the number of charge pump capacitors. When the circuit supplies current to a load (e.g., gate electrodes of a number of MOS FETs), the output voltage level will be reduced from the no-load value by an amount determined by the load current and the operating frequency (charge switching frequency). The higher the operating frequency, the closer will the output voltage approach the no-load value. In the following, the maximum amount of current that can be supplied from such a charge pump circuit while maintaining a predetermined sufficiently high value of stepped-up voltage will be referred to as the drive capability.
With the electronic control apparatus 100 of FIG. 6, capacitors 24 to 26 are respectively connected between the power source terminal Td and the ground terminal Tg of the driver ICs 101 to 103, with each of these capacitors 24 to 26 corresponding to the capacitor C shown in FIG. 7A.
Furthermore with the example of FIG. 6, each of the driver ICs 101, 102 drives two solenoids, whereas the driver IC 103 drives one solenoid, so that the charge pump circuit 113 of the driver IC 103 can be designed to produce a stepped-up voltage while having a smaller drive capability than the charge pump circuits 111 and 112 of the driver ICs 101 and 102. Hence, whereas capacitors having a relatively large capacitance (for example, tantalum capacitors) would be used as the capacitors 24, 25 of the driver ICs 101 and 102, the capacitor 26 of the driver IC 103 could have a comparatively small value of capacitance, so that a smaller and less expensive device such as a ceramic capacitor could be utilized. Furthermore, assuming that the driver IC 101 switches the current through each of the solenoids L1, L2 with a fixed period, and performs control by varying the duty ratio of the switching, the charge pump circuit 111 will be designed to have a large drive capability, so that the level of current flow into the power source terminal Td will be large. Hence, in order to effectively suppress variations in current and voltage appearing on the common power source line 28, the inductor 27 is connected between the common power source line 28 and the power source terminal Td of the driver IC 101. In that way, appropriate noise suppression elements are provided for each of the driver ICs 101 to 103 respectively, in accordance with the particular utilization conditions of these driver ICs.
Thus, with the electronic control apparatus 100 described above, it is necessary to provide respective noise suppression elements for each of the driver ICs 101 to 103, due to the operation of the charge pump circuits 111 to 113 incorporated in these driver ICs. As a result, it is necessary to provide a large number of these noise suppression elements, so that the total number of circuit components becomes large and the manufacturing cost is increased.